1. Field of the Invention
The present invention relates to a control apparatus for controlling a continuously variable transmission for an automotive vehicle.
2. Discussion of the Prior Art
For a motor vehicle, there is known a continuously variable transmission which is adapted to transmit power from an engine to drive wheels, at a continuously variable speed ratio of the transmission. Usually, the speed ratio of such a continuously variable transmission is controlled so as to coincide with a desired or optimum value which is determined based on an amount of operation of an accelerator pedal (amount of opening of a throttle valve of the engine) and a running speed of the vehicle. In this arrangement, the transmission cannot be sufficiently shifted down to the position of the lowest speed ratio where the vehicle is stopped with an abrupt or sudden brake being applied to the wheels of the vehicle. In this case, the vehicle cannot be easily or smoothly re-started. In view of this drawback, laid-open Publication No. 58-170958 of unexamined Japanese Patent Application proposes a control apparatus which is adapted to initiate a shift-down operation of the continuously variable transmission upon reception of a signal indicative of the operation of a brake pedal, so that the speed ratio of the transmission can be lowered to the lowest value with improved reliability. This control apparatus uses the operation of the brake pedal as a signal which indicates a need for applying an engine brake to the vehicle. Namely, the control apparatus commands the transmission to be shifted down toward the position of the lowest speed ratio, to apply the engine brake to the vehicle when the apparatus receives the signal indicative of the operation of the brake pedal. However, this arrangement always causes the transmission to be shifted down to the position of the lowest speed ratio, irrespective of the amount of operation of the brake pedal, even when the vehicle driver who has depressed the brake pedal does not want such a shift-down action of the transmission. Accordingly, the above arrangement may result in an unexpected shift-down operation of the transmission, which deteriorates the driving comfort when the brake pedal is operated.
On the other hand, laid-open Publication No. 62-122834 proposes a control apparatus wherein the commanded optimum speed ratio of the continuously variable transmission or optimum speed of the engine is changed to a greater extent than usual, depending upon a deceleration value of the vehicle when brake is applied to the vehicle. This control arrangement permits a relatively rapid shift-down operation of the transmission, upon brake application to the vehicle, since a control error between the actual and optimum speed ratio values becomes larger than usual.
In the above arrangement, upon detection of brake application to the vehicle, the optimum speed of the engine is increased to thereby reduce the optimum speed ratio of the transmission, for positively increasing the speed ratio control error of the transmission, so that the continuously variable transmission is rapidly shifted down to the position of the lowest speed ratio. Consequently, engine braking is more or less applied to the vehicle, deteriorating the driving comfort of the vehicle. Where the vehicle is equipped with an anti-lock or anti-skid braking system, the rapid shift-down of the transmission to establish the lowest speed ratio will prevent the speeds of the abruptly braked wheels from rising back to the level prior to the braking, even after the anti-lock braking is released.
Where the continuously variable transmission is rapidly shifted down to the position of the lowest speed ratio only when an abrupt braking is applied to the vehicle, the shift-down operation of the transmission will not occur when a light braking is applied to the vehicle while the vehicle is running up an uphill road, for example. In this case, the vehicle may be stopped, but the transmission may not be shifted down to the lowest speed ratio position, and cannot be easily re-started.